Many methods have been used and still others proposed for covering wooden roof decks by metal sheets, pans, or the like. For example, it is well known to use sheet metal overlaid in a manner similar to the overlaying of wooden or asphalt shingles, nailing the individual sheets under the overlap of the adjacent sheets to lessen water leakage and rusting. Roofs of this type are especially vulnerable to high winds which cause curling of the sheets, exposing the nails to rust and leakage.
To overcome these pitfalls, metal sheeting with various interlocking joint constructions have been proposed. These interlocking constructions include forming adjacent upstanding portions on the edges of sheet metal pans and covering these adjacent upstanding portions with clips or the like to prevent leakage. However, leakage will continue to occur in many of these constructions by capillary action between the closely spaced surfaces of the upstanding portions. This leakage is promoted by the freezing and thawing of the air, water, and water vapor trapped between the layers.
A further defect in the prior art methods has been the use of clips which, when forced over upturned pan edges or flanges or upturned flanges and pins, will cause the pin and pan edges to crush, providing excellent gaps for leakage. Still further defects include the expansion and contraction of clips or the like covering devices loosening the clip which then becomes vulnerable to the wind. The present invention overcomes these defects.